During the study of biological samples, it is common to have samples that contain cells of particular interest within a mixture of cells. It may be useful to enrich the sample with respect to the cells of interest in the population for further processing and analysis.
This capability is of interest within the research area to provide an understanding of mechanisms of a cellular process, where the technique has applications for example in the study of cell differentiation, cell lineage and fate, of system development, in the area of regulation, in the response to any external stimulus, to provide new knowledge and understanding. This capability is also of interest for the diagnosis of a disease or disorder (detecting presence of a condition, evaluating the extent of a condition) by comparing the results with normals; and for evaluating the effectiveness of a treatment by looking at changes over time.
Typically, a particular cell type exists within a heterogeneous population of multiple cell types. Examples include differentiated cells within a tissue sample and stem cells [So 2004]; cancer cells and blood vessels within a tissue sample; neurons growing in a substrate of supporting cells; fetal cells in maternal blood; white blood cells of various types (natural killer cells, etc) in blood samples; metastatic cancer cells in tissues samples, etc. In addition a particular cell type may itself consist of a heterogeneous population with respect to some particular characteristic. For example, the cells may consist of subgroups of cells at different positions in the cell cycle, including being quiescent (non-dividing). In addition, the cells may be responding to the environment in different ways or be activated in different ways, and thus expressing different proteins or molecules of interest or entire processes or pathways.
One current approach to sort cells is to use a flow cytometer or a Fluorescence Activated Cell Sorter (FACS) machine. The practice is to label the cells with one or more fluorescent labels, to disperse the cells in a suitable carrier medium, and to pass that carrier medium containing the cells through the FACS machine, in which the carrier is formed into droplets by passing through a nozzle. The resulting droplet is exposed to a beam of excitation light, causing the fluorescent label in an individual cell to fluoresce and emit light which is detected in the form of a signal, which is used to form the basis of a classification decision to accept or reject the cell, and divert it into a receptacle for further processing. Multiple labels can be used to determine various states of the cell and thus make more complex classification decisions. Such an approach is able to sort cells at modest rates, with good discriminating ability when the fluorescent label has the necessary sensitivity and selectivity.
However this approach suffers from a number of drawbacks. Sample preparation and rough handling may perturb the sample and limits it to a single passage. This will be a problem if the cells exist in a supporting matrix and/or the interaction between cells is to be maintained so that only non-adherent cells are used. In addition, since the FACS process involves some delay, this means that some fine-grained temporal and kinetic information is lost making some studies very difficult eg synchronization of cell cycle. In addition, the data collected from each cell is of a fairly coarse nature, making it difficult to distinguish between distinct spatial organizations of the label, due e.g. to translocation processes.
In an alternative approach, the sample may be analysed manually under a microscope with a micro-dissecting stage. Markers (stains) may be added to allow different cell types, components and structures to be visualized, distinguished and manipulated. This is slow, laborious and error-prone, and it is difficult to prevent cross-contamination between sample components or the use of live objects. It is not feasible if the cell type is rare.
In a further technique, a laser may be used for micro-dissection and to cut around the cells of interest [Wittke 2005; Schuetze 1997]. This helps to prevent contamination problems. Alternatively, optical tweezers may be used to displace small scale biological objects, but cannot effectively perform certain types of operation such as the cutting or removal of undesired attached material. Laser ablation can also be used to remove unwanted material.
In an another approach, the sample may be lysed and probed for particular molecules or combinations of molecules such as DNA, RNA or proteins using a microarray. This approach can be very sensitive and selective. However it can be quite slow, preventing fine temporal information from being accessed, and it is generally destructive of the cell under study, thus preventing the collection and further use of cells of a particular type.